The present invention relates to spinal fixation systems and, in particular, to a device for connecting a longitudinal support to a bone anchor.
Several devices for connecting pedicle screws to longitudinal supports for purposes of vertebral fixation are known in the art. These devices offer the advantage that the individual pedicle screws can be affixed to and removed from the longitudinal support at any time without requiring disassembly of the entire fixation system. In addition, the locked relationship between the head of the pedicle screw and the connection device can be maintained while adjusting the position of the connection device relative to the longitudinal support.
One such connection device is disclosed in U.S. Pat. No. 5,584,834 to Errico et al. The Errico patent discloses a coupling element that provides a simple connection between a pedicle screw and a longitudinal support rod while maintaining considerable angular freedom between the two components. Essentially, the coupling element consists of a generally cylindrical body defining an interior chamber for receiving a head of a screw at its lower portion and an external thread on its upper portion. Slots are formed in the external surface of the lower portion so that the interior chamber may resiliently expand to receive the screw head. The intermediate portion of the cylindrical body includes a side receiving channel for receiving the longitudinal support. A locking collar and a securing sleeve both slide over the cylindrical body and a locking nut is threaded on the external thread of the cylindrical body. The longitudinal support may be inserted through the side receiving channel between the locking collar and the securing sleeve. To lock the coupling element, the locking nut is tightened to provide a downward force on the securing sleeve, longitudinal support rod, and ultimately the locking collar, which compresses the interior chamber and thereby locks the position of the screw.
One disadvantage of the coupling element disclosed in the Errico patent is that the cylindrical body is bent upward when the locking nut is being tightened because of the geometry of the side receiving channel. Consequently, the tightening force applied to the longitudinal support is insufficient and the cylindrical body may elastically deform or the locking collar may warp because of uneven pressure distributed on it. When the cylindrical body bends upward, the longitudinal support tends to slip out of the side receiving channel and consequently the securing sleeve bends up too. This problem can only be solved by fitting the locking collar with a guide groove for partially receiving the longitudinal support. This groove, however, requires aligning the locking collar during assembly and creates a risk of assembly errors. Moreover, with regard to pressing the locking collar over the lower portion of the cylindrical body to lock the ball-head of the bone screw, there is a risk of warping the lower portion if the surfaces of the lower portion and the locking collar have imprecisely matching surfaces. Also, the geometry of the lower portion may change during tightening of the tightening nut and adversely affect the locking relationship between the coupling element and the ball-head of the screw.
It is therefore desirable to provide a device for connecting a longitudinal support to a bone anchor that distributes locking forces over the device and longitudinal support in a plane perpendicular to a central axis of the locking collar, prevents warping of the cylindrical body and the locking collar, and prevents the longitudinal support from bending upward and slipping out of the side receiving channel.
The present invention is directed to a device for connecting a longitudinal support to a bone anchor having a rounded head. According to one embodiment, the device includes a body defining a chamber for receiving the rounded head of the bone anchor and a first channel for receiving the longitudinal support. Further, a first sleeve is slidable over the body for compressing the chamber, a second sleeve is slidable over the body for biasing the longitudinal support against the first sleeve at first and second contact points or zones, and a fastener is operatively associated with the body for biasing the second sleeve toward the first sleeve. One of the sleeves includes at least one extended portion for contacting the other sleeve at at least one additional contact point or zone such that forces exerted on the second sleeve by the fastener are transferred to the first sleeve through the at least three contact points or zones. The contact points or zones may be radially displaced about a central axis of the body such that a resultant force exerted by the first sleeve and the longitudinal support on the second sleeve is applied substantially at the central axis.
According to another embodiment, the portion of the body defining the chamber has an outer surface with a convex cross-section. For example, the portion of the body may have a substantially spherical outer surface. Contact between an inner wall of the first sleeve and the outer surface may be along a substantially circular contact zone.
According to yet another embodiment, the body defines an annular recess, and the first sleeve includes a protrusion received in the annular recess. The protrusion limits sliding of the first sleeve on the body such that the chamber cannot expand sufficiently to allow the rounded head of the bone anchor to be removed from the chamber.